Portable navigation devices (PNDs) that include GNSS signal reception and processing functionality are well known and are widely employed as in-car or other vehicle navigation systems.
In general terms, a modern PNDs comprises a processor, memory (at least one of volatile and non-volatile, and commonly both), and map data stored within said memory. The processor and memory cooperate to provide an execution environment in which a software operating system may be established, and additionally it is commonplace for one or more additional software programs to be provided to enable the functionality of the PND to be controlled, and to provide various other functions.
Typically these devices further comprise one or more input interfaces that allow a user to interact with and control the device, and one or more output interfaces by means of which information may be relayed to the user. Illustrative examples of output interfaces include a visual display and a speaker for audible output. Illustrative examples of input interfaces include one or more physical buttons to control on/off operation or other features of the device (which buttons need not necessarily be on the device itself but could be on a steering wheel if the device is built into a vehicle), and a microphone for detecting user speech. In a particularly preferred arrangement the output interface display may be configured as a touch sensitive display (by means of a touch sensitive overlay or otherwise) to additionally provide an input interface by means of which a user can operate the device by touch.
Devices of this type will also often include one or more physical connector interfaces by means of which power and optionally data signals can be transmitted to and received from the device, and optionally one or more wireless transmitters/receivers to allow communication over cellular telecommunications and other signal and data networks, for example Wi-Fi, Wi-Max GSM, CDMA and the like.
PND devices of this type also include a GNSS antenna by means of which satellite-broadcast signals, including location positioning data, can be received and subsequently processed to determine a current location of the device.
The PND device may also include electronic gyroscopes and accelerometers which produce signals that can be processed to determine the current angular and linear acceleration, and in turn, and in conjunction with location information derived from the GNSS signal, velocity and relative displacement of the device and thus the vehicle in which it is mounted. Typically such features are most commonly provided in in-vehicle navigation systems, but may also be provided in PND devices if it is expedient to do so.
The utility of such PNDs is manifested primarily in their ability to determine a route between a first location (typically a start or current location) and a second location (typically a destination). These locations can be input by a user of the device, by any of a wide variety of different methods, for example by postcode, street name and house number, previously stored “well known” destinations (such as famous locations, municipal locations (such as sports grounds or swimming baths) or other points of interest), and favourite or recently visited destinations.
Typically, the PND is enabled by software for computing a “best” or “optimum” route between the start and destination address locations from the map data. A “best” or “optimum” route is determined on the basis of predetermined criteria and need not necessarily be the fastest or shortest route. The selection of the route along which to guide the driver can be very sophisticated, and the selected route may take into account existing, predicted and dynamically and/or wirelessly received traffic and road information, historical information about road speeds, and the driver's own preferences for the factors determining road choice (for example the driver may specify that the route should not include motorways or toll roads).
In addition, the device may continually monitor road and traffic conditions, and offer to or choose to change the route over which the remainder of the journey is to be made due to changed conditions. Real time traffic monitoring systems, based on various technologies (e.g. mobile phone data exchanges, fixed cameras, GPS fleet tracking) are being used to identify traffic delays and to feed the information into notification systems.
PNDs of this type may typically be mounted on the dashboard or windscreen of a vehicle, but may also be formed as part of an on-board computer of the vehicle radio or indeed as part of the control system of the vehicle itself. The navigation device may also be part of a hand-held system, such as a PDA (Portable Digital Assistant) a media player, a mobile phone or the like, and in these cases, the normal functionality of the hand-held system is extended by means of the installation of software on the device to perform both route calculation and navigation along a calculated route.
Route planning and navigation functionality may also be provided by a desktop or mobile computing resource running appropriate software. For example, the Royal Automobile Club (RAC) provides an on-line route planning and navigation facility at http://www.rac.co.uk, which facility allows a user to enter a start point and a destination whereupon the server to which the user's PC is connected calculates a route (aspects of which may be user specified), generates a map, and generates a set of exhaustive navigation instructions for guiding the user from the selected start point to the selected destination. The facility also provides for pseudo three-dimensional rendering of a calculated route, and route preview functionality which simulates a user travelling along the route and thereby provides the user with a preview of the calculated route.
In the context of a PND, once a route has been calculated, the user interacts with the navigation device to select the desired calculated route, optionally from a list of proposed routes. Optionally, the user may intervene in, or guide the route selection process, for example by specifying that certain routes, roads, locations or criteria are to be avoided or are mandatory for a particular journey. The route calculation aspect of the PND forms one primary function, and navigation along such a route is another primary function.
During navigation along a calculated route, it is usual for such PNDs to provide visual and/or audible instructions to guide the user along a chosen route to the end of that route, i.e. the desired destination. It is also usual for PNDs to display map information on-screen during the navigation, such information regularly being updated on-screen so that the map information displayed is representative of the current location of the device, and thus of the user or user's vehicle if the device is being used for in-vehicle navigation.
An icon displayed on-screen typically denotes the current device location, and is centred with the map information of current and surrounding roads in the vicinity of the current device location and other map features also being displayed, determined by the PND using a GNSS receiver. Additionally, navigation information may be displayed, optionally in a status bar above, below or to one side of the displayed map information, examples of navigation information include a distance to the next deviation from the current road required to be taken by the user, the nature of that deviation possibly being represented by a further icon suggestive of the particular type of deviation, for example a left or right turn. The navigation function also determines the content, duration and timing of audible instructions by means of which the user can be guided along the route. As can be appreciated a simple instruction such as “turn left in 100 m” requires significant processing and analysis. As previously mentioned, user interaction with the device may be by a touch screen, or additionally or alternately by steering column mounted remote control, by voice activation or by any other suitable method.
A further important function provided by the device is automatic route re-calculation in the event that: a user deviates from the previously calculated route during navigation (either by accident or intentionally); real-time traffic conditions dictate that an alternative route would be more expedient and the device is suitably enabled to recognize such conditions automatically, or if a user actively causes the device to perform route re-calculation for any reason.
It is also known to allow a route to be calculated with user defined criteria; for example, the user may prefer a scenic route to be calculated by the device, or may wish to avoid any roads on which traffic congestion is likely, expected or currently prevailing. The device software would then calculate various routes and weigh more favourably those that include along their route the highest number of points of interest (known as POIs) tagged as being for example of scenic beauty, or, using stored information indicative of prevailing traffic conditions on particular roads, order the calculated routes in terms of a level of likely congestion or delay on account thereof. Other POI-based and traffic information-based route calculation and navigation criteria are also possible.
Although the route calculation and navigation functions are fundamental to the overall utility of PNDs, it is possible to use the device purely for information display, or “free-driving”, in which only map information relevant to the current device location is displayed, and in which no route has been calculated and no navigation is currently being performed by the device. Such a mode of operation is often applicable when the user already knows the route along which it is desired to travel and does not require navigation assistance.
Devices of the type described above, for example the 720T model manufactured and supplied by TomTom International B.V., provide a reliable means for enabling users to navigate from one position to another.
The above-described functionality of the PND requires the PND to determine a position fix using the GNSS receiver. Accordingly, on start up of a PND, the GNSS software establishes a position fix for the device. Delays in obtaining a position fix will result in delays in the PND carrying out/being enabled to carry out the specified functionality, such as determine a navigable route. In certain circumstances it can take a long time to establish a first position fix (time to first fix (TTFF)), such as between several minutes and 1 hour. This problem is particular pronounced when the PND has been switched off and relocated by a great distance before being switched back on (far start), for example, if a user takes the PND with him/her on a long haul flight or during delivery of the PND to a customer.
Referring to FIG. 16, the signal processing for a GNSS receiver is based on channelized architecture. Before allocating a channel to a specific satellite (PRN code), the receiver must know which satellites are currently visible. There are two common operating modes for a GNSS receiver to find the visible satellites. One is referred to as cold start and the other is referred to as warm start.
GNSS receivers typically comprise an almanac containing information on the GNSS satellites, such as satellite status and orbital information. In a warm start, the GNSS receiver combines the information in the stored almanac with the last position computed by the GNSS receiver to compute course positions (i.e. Doppler shift) of all satellites since the PND was switched off and determine the satellites that should be visible at the time the PND is switched on. However, if the PND has been moved a significant distance away from the position it was in when it was turned off, the position information cannot be trusted. For example, if during the time the PND is switched off, the PND is relocated from London to Taipei, when the PND is switched back on, the satellite constellation visible to the GNSS receiver in Taipei will be different to the satellite constellation predicted from the information stored in the almanac.
In a cold start, the receiver does not rely on the information stored in the almanac but searches for the visible satellites from scratch. Such a search can take significant time.
Attempts have been made to reduce the TTFF.
One solution implemented by some GPS providers, for example Broadcom's BCM4750, takes advantage of a 24-channel GPS receiver that searches for all GPS satellites simultaneously regardless of operation modes (i.e. warm start or cold start). However, a receiver having such a high number of channels has increased hardware complexity and power consumption (a rule of thumb is that one channel consumes 1-2 mA during satellite acquisition). Greater hardware complexity will result in greater cost for the GPS receiver. Furthermore, to implement such a solution in known GNSS receivers equipped with 12 to 16 channels requires modification of the hardware (through design and tapeout of a new chip) and cannot be achieved through software/firmware upgrades. For traditional GPS, a 24-channel receiver is sufficient but for future GNSS systems (such as Galileo, GLOSNASS, modernized GPS) a receiver may require many more channels than 24 to be able to search for all satellites simultaneously.
A system developed by Qualcomm, see International Patent Application WO 2006/102508, uses Mobile Country Code information (MCC) to reduce the TTFF in far start cases. MCCs are transmitted by cellular networks and can be used by a GNSS device to identify the country/territory the GNSS device is in and therefore, the location of the GNSS device, even during a far start. Use of MCC can result in a reduction in the TTFF during a far start for most countries, but in some countries with large territories (e.g Russian Federation, United States of America, Canada, People's Republic of China and Republic of Chile) the use of MCC is less effective or may even result in an increase in the TTFF.
A further solution is A-GPS which downloads ephemeris and other information that can be used as an aid to find visible satellites over a cellular network from either the cellular network provider (control plane, CP) or a service content provider (user plane, UP). The drawback of such a technique is that for far start scenarios, the mobile telephone or other cellular device used by the GNSS receiver is likely to be in roaming mode (operating within the cellular network (a visited network) in which the cellular device was not originally registered) and therefore, the downloading of the information is likely to incur significant cost. Furthermore, in the case that the information is provided by the cellular network provider, the home network and the visited network must both be compatible to allow the GNSS receiver to obtain this information and for current cellular networks such compatibility between the networks is not common.
Other GPS receivers automatically reset to a blind search mode when the searching time based on a calculated satellite constellation exceeds a predetermined threshold.